The rotor has a claw structure comprising magnetic assemblies and an annular excitation winding placed around the rotational shaft of the machine. The rotor consists of two magnet wheels, each comprising claws, each of which is inserted between two claws of the magnet wheel opposite. When the excitation winding is supplied electrically, the magnet wheels, which are advantageously made of ferromagnetic material, are magnetised, and south poles are thus created at the level of the claws of one of the magnet wheels, and north poles are creased at the level of the claws of the other magnet wheel.
The magnetic assemblies comprise at least one permanent magnet, and are disposed between two claws. They are used in particular to prevent leakages of the magnetic flux between the magnetic poles in the form of a claw. In fact, it has been determined that a substantial part of the magnetic flux created by the winding of the rotor passes via leakage paths, instead of passing through the air gap of the machine, and giving rise to the required induction in the poles of the stator. For this purpose, according to solutions of the prior art, magnets are placed in pairs between at least some of the consecutive claws of the magnet wheels, either by means of clips or the like, or by means of two grooves provided in one of the lateral edges of the claws between which the magnet is situated. In the second case, the magnets must fill all of the interpolar space, which increases the cost of the machine, since these magnets are expensive.
In the field, a rotor for a rotary electrical machine is already known which comprises two magnet wheels comprising a series of claws with axial orientation and a globally trapezoidal form, which extend axially from a radial projection of the outer radial end edge of the said magnet wheel, in the direction of the other magnet wheel, such that each claw of a magnet wheel is situated in the space which exists between two consecutive claws of the other magnet wheel, and comprises at least one magnetic assembly provided with at least one interpolar magnet in the interpolar space defined between a first claw of a first magnet wheel and a second claw of a second magnet wheel, the said magnet comprising laterally two first faces which are delimited by first and second free ends, the said two first faces extending respectively along first and second claws; each of the first and second claws having a head end and a lateral face, such that the adjacent lateral face is opposite, the said lateral face comprising third and fourth opposite ends between which the magnet is in contact.
A rotor of this type for a rotary electrical machine is disclosed in FR 2 793 085.
In a rotor of this type, it has been found that part of the magnetic flux created by the winding of the rotor passed via leakage paths instead of passing through the air gap defined between the outer periphery of the rotor and the inner periphery of the stator of the electrical machine. More specifically, as shown in FIG. 1a which shows a rotor according to the prior art, the magnetic assembly is arranged relative to the first and second claws such that one of the free ends of the said magnetic assembly is recessed relative to the head end of one of the two claws. In addition, the lateral faces of each of the first and second claws define a cross-section which decreases linearly, irrespective of the proximity of the magnet which is adjacent to them.
As represented in FIG. 1a in which the reference 1 designates the claw rotor, and the reference 2 designates the rotor shaft, these leakages of magnetic flux take place in the location of the interpolar space, in the vicinity of the free end of the magnetic assembly, which is recessed relative to the head end, or free end, of one of the two claws. The leakages are caused mainly by the fact that part of the interpolar space is left free, thus putting the lateral faces of the first and second claws directly opposite one another. In addition, the geometry of the head end, in the vicinity of the magnetic assembly, also assists these leakages of magnetic flux, since the lateral face, in the location of the head end, is close to the magnetic assembly and to the other claw which is opposite it. In this FIG. 1a, 119 shows the chamfers for connection of the foot of the claw with axial orientation (with no reference) to the radial projection (with no reference) derived from the edge of the flange of the magnet wheel concerned. The claws with axial orientation form together with their associated radial projection claws themselves, with the spaces between the radial projections being globally in the form of a “V” with a flat top. For further details reference will be made to document FR 2 676 873, and in particular to FIGS. 2 and 3 of this document. Since the magnet wheels are similar to those in this document FR 2 676 873, the anti-noise chamfers of each magnet wheel at the level of the chamfer 119 have been given the reference 120.
In order to prevent these leakages, other solutions according to the prior art, as represented in FIG. 1b, propose a solution wherein the magnetic assembly fills the interpolar space entirely. A solution of this type is satisfactory for preventing leakages of magnetic flux, but is not satisfactory from an economical point of view.
In a competitive industrial context, it is important for the technical solutions to be as economical as possible. However the magnetic assembly comprising an interpolar magnet is an object made of a material which is increasingly expensive, all the more so since it generally involves a rare earth magnet, the cost of which is increasing continually. In these conditions, in order to limit the costs, it appears necessary to limit the size of the magnets.